What slackjaw told you that?
The Lee conical bullet is made in two diameters. As a result, it comes in two weights. Both have heels made to slip into the chamber.
The .450" Lee conical weighs 200 grs. when cast of pure lead, and is for Remington and Colt designs.
The .456" Lee conical weighs 220 grs. when cast of pure lead, and is for the Ruger Old Army. Some of the Walkers and Dragoons may have larger chamber mouths, and require this bullet for a tight fit.
The Lee conical bullet is designed with a bearing band slightly smaller than the ogive of the bullet. This means that, when seated in the chamber, the bearing band stops against the chamber mouth. The bullet diameter, just ahead of the bearing band, is even larger.
So, as you seat the bullet the bearing band provides some resistance as it's swaged into the chamber, then the bullet offers more resistance as its seated. In effect, this gives you two sealed bands, with a groove filled with grease between them.
The different sized bands (bearing band and bullet) help to align the bullet straight. The biggest problem that cap and ball shooters have, when using conicals, is keeping the bullet straight as it's pushed into the chamber. This is where the heel is invaluable.
Shooters new to this style of gun often think that they can use standard lead pistol bullets in their cap and ball, but without a heel to start the bullet straight into the chamber this is almost certainly impossible. Standard, plain-based bullets get cocked one way or the other during seating. The go down the rifled bore not quite point-on, and emerge spinning erratically, out of balance. Accuracy suffers.
Shooters need to remember that a bullet is essentially a toy top. If its rotation is the least bit off, the top wobbles. Bullets do the same thing, and accuracy suffers.
I've been shooting cap and ball revolvers since about 1970. The Lee is the most accurate conical bullet I've found. I've tried various designs: traditional, authentic, Buffalo, Taurus (back in the 1970s), Lyman, etc. The Lee tops all, but it's still not as accurate as the plain ol' lead ball.
Conical bullets are fun to play with, but they're much more work than a ball. I use them occasionally, but for convenience and accuracy nothing beats a proper-sized ball of soft lead.